Catalyst deterioration diagnosis method

ABSTRACT

A method for diagnosing the degree of deterioration of a catalyst disposed in an exhaust path of an internal combustion engine and oxidizes or adsorbs a target gas, including at least one of a hydrocarbon gas and a carbon monoxide gas, in an exhaust gas from the internal combustion engine, is adapted to determine whether deterioration exceeding an acceptable level of a catalyst occurs or not by comparing, at any timing when the internal combustion engine is in a state of a steady operation, the concentration of a target gas detected downstream from the catalyst in the exhaust path with a threshold value of the concentration of a target gas corresponding to the temperature of a catalyst at the timing which is previously defined according to an allowable range of an index value representing the degree of oxidation or adsorption at the catalyst corresponding to the temperature of a catalyst at the timing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for diagnosing the degree ofdeterioration of a catalyst oxidizing or adsorbing an unburnedhydrocarbon gas.

2. Description of the Background Art

Conventionally, concerning OBD (function diagnosis) of an exhaust gaspurification system for automobile, that is, a function of determiningwhether a catalyst works normally or not in the system, many patentapplications are filed centered of automotive companies; however, mostof them are techniques using an exhaust gas temperature sensor, anoxygen sensor, a wide range oxygen concentration sensor (λ sensor), aNOx sensor, and a PM sensor, and a diagnosis target is a three-waycatalyst, an oxidation catalyst, a NOx storage catalyst, a NOx selectivereduction catalyst, and a diesel particulate filter (DPF) (e.g.,Japanese Patent Application Laid-Open No. 2001-263048, Japanese PatentApplication Laid-Open No. 2005-240716, Japanese Patent ApplicationLaid-Open No. 2012-036860, Japanese Patent Application Laid-Open No.2012-241594, and Japanese Patent Application Laid-Open No. 07-103039(1995)).

On the other hand, as a hydrocarbon gas sensor (HC sensor) forautomobile exhaust gas measurement, sensors of various principles andvarious types are researched and developed. For example, a semiconductortype HC sensor which is applicable for purification control of NOx in azeolite catalyst and can selectively detect HC (hydrocarbon) having alarge molecular weight, is already publicly known (e.g., refer toJapanese Patent No. 2876793). As other HC sensors, sensors of acatalytic-combustion type, an oxygen-concentration difference detectiontype, a limiting current type, a mixed-potential type and the like arewidely known, and a large number of patent applications have been filedat the peak of around year 2000.

It is supposed that these HC sensors are mainly used for purificationperformance inspection of exhaust gas purification devices (TWC:three-way catalyst, NSC: NOx storage catalyst) to be mounted on agasoline engine of theoretical equivalent ratio (air excess ratio λ=1)combustion or lean burn (lean burn type, λ>1), or the control ofinjection amount of unburned hydrocarbon in a diesel engine.

Entering the 2010s, exhaust gas regulations are enhanced in NorthAmerica, and among others, OBD of an oxidation catalyst to diesel enginevehicles will be obliged in the future. Specifically, the need forperforming OBD targeted at NMHC (Non Methane HydroCarbon) in O₂(oxygen)-excess atmosphere which is an exhaust gas atmosphere of thediesel engine against an oxidation catalyst for a diesel engine, hasarisen.

However, there is a problem that as disclosed in Japanese PatentApplication Laid-Open No. 2001-263048, Japanese Patent ApplicationLaid-Open No. 2005-240716, Japanese Patent Application Laid-Open No.2012-036860, Japanese Patent Application Laid-Open No. 2012-241594 andJapanese Patent Application Laid-Open No. 07-103039 (1995), a techniquesusing publicly known sensor cannot respond to such OBD or can onlyindirectly diagnose.

For example, Japanese Patent Application Laid-Open No. 2001-263048discloses a technique of using a relation that when the capability toconvert (oxidize or combust) unburned hydrocarbon in an oxidationcatalyst is deteriorated, heat generation energy is also lowered.Schematically describing, in this technique, a temperature difference ΔTgenerated at the time of fuel injection in exhaust gas temperaturesensors arranged upstream and downstream of an oxidation catalyst in anexhaust path is measured, and from the measured values, a degree ofdeterioration of capability to convert (oxidize or combust) unburnedhydrocarbon in the oxidation catalyst is indirectly diagnosed.

However, in such a method, there is a problem that error cause due tochanges in an exhaust gas temperature and exhaust gas flow rate inactual use is too large, and a problem that a fuel injection amount foraccelerating heat generation is large and therefore deterioration offuel economy cannot be avoided.

Further, Japanese Patent Application Laid-Open No. 2005-240716 disclosesa technique of using the fact that when the capability to convertunburned hydrocarbon in an oxidation catalyst is deteriorated,consumption of oxygen during oxidative combustion is changed.Schematically describing, this is a technique in which a degree ofdeterioration of capability to convert unburned hydrocarbon in theoxidation catalyst is indirectly diagnosed, based on a difference ΔXbetween output values XF and AR of two wide-range oxygen concentrationsensors (λ sensor) arranged upstream and downstream of an oxidationcatalyst in an exhaust path, or from a change in value of oxygenconsumption in the oxidation catalyst measured based on a differencebetween output values (electromotive force values) of two oxygensensors.

However, the oxygen concentration in the diesel exhaust gas being anO₂-excessive atmosphere is about 10% (=100000 ppm), whereas an amount(concentration) of hydrocarbon which the oxidation catalyst converts(oxidizes or combusts) is usually about several hundreds ppm, and anamount (concentration) of oxygen to be consumed in burning such a slightamount of hydrocarbon is also only about several hundreds ppm at most.That is, this means that in order to diagnose the deterioration of anoxidation catalyst by use of the air-fuel ratio sensor or the oxygensensor, it is necessary to calculate, with accuracy, Δλ or a differencein electromotive force which corresponds to a change in an oxygenconsumption of ppm order, but measurement accuracy of such an extentcannot be attained in the first place in the air-fuel ratio sensor andthe oxygen sensor.

Japanese Patent Application Laid-Open No. 2012-036860 discloses atechnique of determining a deterioration degree of an oxidationcatalyst, which oxidizes NO to NO₂, based on an output value(electromotive force value) of a NOx sensor arranged downstream of theoxidation catalyst in an exhaust path, and a map previously defined.

However, even though capability to oxidize NO of the oxidation catalystcan be diagnosed by such a technique, the result of such diagnosiscannot be applied to the diagnosis of the capability to convert (oxidizeor combust) unburned hydrocarbon. The reason for this is that sincefunctions of a noble metal catalyst and a storage material vary withvarious gases (for example, HC, CO, NO, etc.), a relation between theexhaust gas temperature and the conversion rate (oxidizing capability)in each gas varies and there is not a definite correlation between them.

In addition, an estimated value is used for NOx value in the exhaust gasimmediately after discharge from engine, and factors other than anengine speed and an engine load are not considered in setting such anestimated value, and therefore estimation accuracy may be significantlylowered depending on usage conditions.

Further, Japanese Patent Application Laid-Open No. 2012-241594 disclosesa method in which an exhaust gas temperature sensor and a λ sensor arearranged upstream and downstream of an oxidation catalyst, and adeterioration degree of the oxidation catalyst is diagnosed based on arequired oxygen amount obtained from an estimated value of HC storageamount at the time of normal state of the oxidation catalyst, and anestimated value of an actual oxygen consumption being an oxygen amountactually consumed by the oxidation catalyst which is computed based onan output value of the sensor.

However, in such a method, the diagnosis is only performed based onestimated values, and there is a problem that an influence of error of asignal from each sensor cannot be avoided and the diagnosis accuracy islow.

Further, Japanese Patent Application Laid-Open No. 07-103039 (1995)discloses an engine system whose target is TWC or NSC of a gasolineengine. Japanese Patent Application Laid-Open No. 07-103039 (1995) doesnot disclose diagnosis of an oxidation catalyst in the diesel exhaustgas in an O₂-excessive state at all.

SUMMARY OF THE INVENTION

The present invention relates to a method for diagnosing a degree ofdeterioration of a catalyst oxidizing or adsorbing an unburnedhydrocarbon gas.

In accordance with the present invention, a catalyst is disposed in anexhaust path of an internal combustion engine and oxidizes or adsorbs atarget gas, including at least one of a hydrocarbon gas and a carbonmonoxide gas, contained in an exhaust gas from the internal combustionengine, and a method for diagnosing the degree of deterioration of thecatalyst comprises the steps of: a) detecting the concentration of thetarget gas downstream from the catalyst in the exhaust path at anytiming when the internal combustion engine is in a state of a steadyoperation; and b) determining whether deterioration exceeding anacceptable level of the catalyst occurs or not by comparing theconcentration of the target gas detected in the step a) with a thresholdvalue of the concentration of the target gas corresponding to thetemperature of the catalyst at the timing, wherein the threshold valueis previously defined according to an allowable range of an index valuerepresenting the degree of oxidation or adsorption at the catalystcorresponding to the temperature of the catalyst at the timing.

In accordance with another aspect of the present invention, a catalystis disposed in an exhaust path of an internal combustion engine andoxidizes or adsorbs a target gas, including at least one of ahydrocarbon gas and a carbon monoxide gas, contained in an exhaust gasfrom the internal combustion engine, and a method for diagnosing thedegree of deterioration of the catalyst comprises the steps of: a)detecting the target gas at the upstream and the downstream from thecatalyst in the exhaust path at any timing when the internal combustionengine is in a state of a steady operation to calculate theconcentration of the target gas; and b) determining whetherdeterioration exceeding an acceptable level of the catalyst occurs ornot by comparing an index value representing the degree of oxidation oradsorption at the catalyst with a threshold value of the index valuecorresponding to the temperature of the catalyst at the timing, based onthe calculated results obtained in the step a).

In accordance with the present invention, it is possible to diagnose, inreal time and with excellent accuracy, the degree of deterioration ofcatalytic ability in an oxidation catalyst, which is disposed at amidpoint in the exhaust pipe from the engine main body being a dieselengine in an engine system, and oxidizes or adsorbs an unburnedhydrocarbon gas in an exhaust gas. It is implemented based on theconcentration of an unburned hydrocarbon gas at a downstream positionclose to the oxidation catalyst which is directly measured by ahydrocarbon gas sensor disposed at the position in the exhaust pipe.

Accordingly, it is an object of the present invention to provide amethod in which the degree of deterioration of an oxidation catalyst canbe diagnosed with accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing a schematic configuration of adiesel engine system 1000 configured to include an oxidation catalystdiagnosis system DS1 of a first preferred embodiment;

FIGS. 2A and 2B are each a schematic sectional view schematicallyshowing an example of a configuration of a HC sensor 100;

FIG. 3 is a view schematically showing a relation between a temperatureand a conversion rate of an oxidation catalyst 600;

FIG. 4 is a view showing output values of a HC sensor 100 for the fresharticle and the aged article, respectively, in flowing actual exhaustgases (containing HC, CO and O₂) as the test gases having theapproximately same unburned hydrocarbon gas concentrations from anupstream side;

FIG. 5 is a graph showing the results at the time when in the detectionby the HC sensor 100 shown in FIG. 4, gas analysis is performed using angas analyzer at the downstream of the HC sensor 100.

FIGS. 6A and 6B are each a view schematically showing another example ofa conversion rate profile Pa;

FIG. 7 is a view showing changes in an output value in the HC sensor 100at the time when an extremely small amount of fuel is injected for ashort time from a fuel injection valve 301 in the middle of an operationunder a certain condition of an engine system 1000 provided with an agedarticle as an oxidation catalyst 600, as well as changes in ahydrocarbon gas concentration in a gas analyzer provided in the vicinityof the HC sensor 100;

FIG. 8 is a view showing an appearance of changes in output value forapplying a ΔT method;

FIG. 9 is a view showing an appearance of changes in output value forapplying a Δλ method;

FIG. 10 is a view showing an example of a procedure of diagnosis basedon Passive OBD.

FIG. 11 is a view showing a relation between the concentration Nu of anunburned hydrocarbon gas on an upstream side and the concentration Nl ofan unburned hydrocarbon gas on a downstream side in several conversionrates in the case where a temperature of an oxidation catalyst 600 is inan oxidation region in which a temperature is higher than T2;

FIG. 12 is a view showing a relation between the concentration Nu of anunburned hydrocarbon gas on an upstream side and the concentration Nl ofan unburned hydrocarbon gas on a downstream side in several conversionrates in the case where a temperature of an oxidation catalyst 600 is inan adsorption region in which a temperature is T1 or lower;

FIG. 13 is a view showing an example of a procedure of diagnosis byActive OBD;

FIG. 14 is a view showing a relation between the concentration Nu of anunburned hydrocarbon gas on an upstream side and the concentration Nl ofan unburned hydrocarbon gas on a downstream side in several conversionrates in the case where a temperature of an oxidation catalyst 600 is inan oxidation region;

FIG. 15 is a view showing a relation between the concentration Nu of anunburned hydrocarbon gas on an upstream side and the concentration Nl ofan unburned hydrocarbon gas on a downstream side in several conversionrates in the case where a temperature of an oxidation catalyst 600 is inan adsorption region;

FIG. 16 is a view illustrating a change of a HC sensor output withrespect to a time in performing diagnosis by Active OBD on a fresharticle and an aged article as targets for the case where a temperatureof the oxidation catalyst 600 is 200° C.;

FIG. 17 is a view illustrating a change of a HC sensor output withrespect to a time in performing diagnosis by Active OBD on a fresharticle and an aged article as targets for the case where a temperatureof the oxidation catalyst 600 is 150° C.;

FIG. 18 is a view showing the results of analyzing components of anexhaust gas G by a gas analyzer in acquiring the HC sensor output shownin FIG. 16 on the fresh article and the aged article;

FIG. 19 is a view schematically showing a schematic configuration of anengine system 2000 configured to include an oxidation catalyst diagnosissystem DS2 of a second preferred embodiment;

FIG. 20 is a view showing an example of a procedure of performingPassive OBD by an oxidation catalyst diagnosis system DS2;

FIG. 21 is a view illustrating a HC sensor output in a fresh article inthe case where a temperature of an oxidation catalyst 600 is 200° C.;and

FIG. 22 is a view illustrating a HC sensor output in an aged article inthe case where a temperature of an oxidation catalyst 600 is 200° C.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Preferred EmbodimentOverview of System

FIG. 1 is a view schematically showing a schematic configuration of adiesel engine system (hereinafter, also referred to simply as an enginesystem) 1000 configured to include an oxidation catalyst diagnosissystem DS1 of first preferred embodiment of the present invention.

The oxidation catalyst diagnosis system DS1 mainly comprises ahydrocarbon gas sensor (hereinafter, also referred to as a HC sensor)100, a temperature sensor 110, and an electronic control device 200being a control device which controls an operation of the whole enginesystem 1000.

The engine system 1000 mainly includes, in addition to the oxidationcatalyst diagnosis system DS1, an engine main body 300 being a dieselengine as a type of internal combustion engine, a plurality of fuelinjection valves 301 which inject fuel to the engine main body 300, afuel-injection instructing part 400 for instructing the fuel injectionvalve 301 to inject fuel, an exhaust pipe 500 forming an exhaust pathwhich discharges the exhaust gas (engine exhaust gas) G generated in theengine main body 300 to the outside, and an oxidation catalyst 600, suchas platinum or palladium, which is disposed at a midpoint in the exhaustpipe 500, and oxidizes or adsorbs an unburned hydrocarbon gas in theexhaust gas G. In the preferred embodiment, in a relative sense, aposition near the engine main body 300 in the exhaust pipe 500, which isone end side of the exhaust pipe 500, is referred to as an upstreamside, and a position near an exhaust port 510 located on a side oppositeto the engine main body 300 is referred to as a downstream side.

The engine system 1000 is typically mounted on an automobile, and insuch a case, the fuel-injection instructing part 400 is an acceleratorpedal.

In the engine system 1000, the electronic control device 200 isconfigured to send a fuel-injection instructing signal sg1 to the fuelinjection valve 301. The fuel-injection instructing signal sg1 isusually sent according to a fuel-injection request signal sg2 which isgiven to the electronic control device 200 from the fuel-injectioninstructing part 400 during the operation (during the driving) of theengine system 1000, and requests injection of a predetermined amount offuel (for example, the accelerator pedal is pressed down to requestoptimal fuel injection in consideration of many parameters such as anaccelerator position, an intake oxygen amount, an engine speed andtorque), and in addition to this, sometimes the fuel-injectioninstructing signal sg1 is sent for the operation of the oxidationcatalyst diagnosis system DS1.

Further, a monitor signal sg3 which monitors various conditions withinthe engine main body 300 is given from the engine main body 300 to theelectronic control device 200.

In the engine system 1000, the exhaust gas G from the engine main body300 being a diesel engine is a gas of an O₂ (oxygen)-excessiveatmosphere having an oxygen concentration of about 10%. Such an exhaustgas G specifically includes nitrogen oxides, soot (graphite) and thelike in addition to oxygen and an unburned hydrocarbon gas. In thespecification, examples of the unburned hydrocarbon gas, which is a gas(target gas) serving as a target of treatment of adsorption or oxidationin the oxidation catalyst 600, include carbon monoxide (CO) in additionto typical hydrocarbon gases (gases classified as hydrocarbon in termsof chemical formula) such as C₂H₄, C₃H₆, and n-C8. Further, the HCsensor 100 can suitably detect a target gas including CO. However, CH₄is excluded from the target gas.

In addition, the engine system 1000 may include one or a plurality ofother purification apparatuses 700 in the midpoint of the exhaust pipe500 in addition to the oxidation catalyst 600.

A diagnosis target of the oxidation catalyst diagnosis system DS1 is thedegree of deterioration of the oxidation catalyst 600 (morespecifically, the degree of deterioration of catalytic ability of theoxidation catalyst 600). The oxidation catalyst 600 is disposed tosuppress flow of an unburned hydrocarbon gas out of an exhaust outlet510 at a tip of the exhaust pipe 500 by the adsorption or oxidation ofthe unburned hydrocarbon gas in the exhaust gas G flown from upstream;however, its catalytic ability (specifically adsorbing ability andoxidizing ability) is deteriorated with time. When such deteriorationoccurs, an amount of the unburned hydrocarbon gas, not captured in theoxidation catalyst 600 and flowing downstream, increases, and it is notpreferred. The oxidation catalyst diagnosis system DS1 of the preferredembodiment is configured to diagnose the degree of deterioration of theoxidation catalyst 600 by detecting an unburned hydrocarbon gas passingthrough the oxidation catalyst 600 by the HC sensor 100.

The oxidation catalyst diagnosis system DS1, as described above,includes the HC sensor 100, and the temperature sensor 110, and theformer is disposed on an downstream side of the oxidation catalyst 600in the exhaust pipe 500, and detects a concentration of an unburnedhydrocarbon gas at this location, and the latter is disposed on anupstream side of the oxidation catalyst 600 and detects a temperature ofthe exhaust gas G (exhaust gas temperature). Each of the HC sensor 100and the temperature sensor 110 is disposed with its one end insertedinto the exhaust pipe 500.

Schematically describing, in the oxidation catalyst diagnosis systemDS1, the electronic control device 200 is configured to determinewhether deterioration occurs in the oxidation catalyst 600 or not, basedon a HC detection signal sg11 sent from the HC sensor 100 and an exhaustgas temperature detection signal sg12 sent from the temperature sensor110. A configuration example of the HC sensor 100 and a detail of thedeterioration diagnosis will be described later. On the other hand, asthe temperature sensor 110, publicly known sensors may be used which areused for measurement of the exhaust gas temperature in common enginesystems.

In addition, the electronic control device 200 has a storage portion,not shown, composed of, for example, memory and HDD, and this storageportion stores threshold data used in diagnosing the degree ofdeterioration of the oxidation catalyst 600 described later as well asprograms controlling the operations of the engine system 1000 and theoxidation catalyst diagnosis system DS1.

<Configuration Example of HC Sensor>

FIGS. 2A and 2B are each a schematic cross-sectional view schematicallyshowing an example of configuration of a HC sensor 100 to be used in thepreferred embodiment. FIG. 2A is a vertical cross-sectional view of asensor element 101 being a main component of the HC sensor 100, which istaken along the longitudinal direction of the sensor element 101. FIG.2B is a view including a cross-section of the sensor element 101perpendicular to the longitudinal direction taken along a position A-A′of FIG. 1A.

The HC sensor 100 used in the preferred embodiment is a so-calledmixed-potential gas sensor. Schematically describing, using a potentialdifference that occurs between a sensing electrode 10, which is providedon the surface of the sensor element 101 mainly composed of ceramicbeing an oxygen-ion conductive solid electrolyte such as zirconia(ZrO₂), and a reference electrode 20, which is provided inside thesensor element 101, due to a difference in the concentration of a gascomponent being a measurement target between the portions near theelectrodes on the basis of the principle of mixed potential, the HC gassensor 100 determines the concentration of the gas component of ameasurement gas.

In the presence of a plurality of unburned hydrocarbon gases in ameasurement gas, a potential difference occurring between the sensingelectrode 10 and the reference electrode 20 has a value reflecting allthe plurality of unburned hydrocarbon gases, and thus, a concentrationvalue to be determined is also a total sum of the concentrations of theplurality of unburned hydrocarbon gases.

The sensor element 101 mainly includes a reference gas introductionlayer 30, a reference gas introduction space 40, and a surfaceprotective layer 50 in addition to the sensing electrode 10 and thereference electrode 20.

In the preferred embodiment, the sensor element 101 has the structure inwhich six layers, namely, a first solid electrolyte layer 1, a secondsolid electrolyte layer 2, a third solid electrolyte layer 3, a fourthsolid electrolyte layer 4, a fifth solid electrolyte layer 5, and asixth solid electrolyte layer 6, each formed of an oxygen-ion conductivesolid electrolyte, are laminated in the stated order from the bottomside of FIGS. 2A and 2B. The sensor element 101 additionally includesother components mainly between those layers or on an outer peripheralsurface of the element. The solid electrolytes constituting those sixlayers are fully airtight. Such a sensor element 101 is manufactured by,for example, laminating ceramic green sheets corresponding to individuallayers, which have been subjected to a predetermined process andprinting of a circuit pattern, and further, by integrating the laminatedlayers through firing.

In the following description, for convenience sake, in FIGS. 2A and 2B,the surface located as the upper surface of the sixth solid electrolytelayer 6 is referred to as a front surface Sa of the sensor element 101,and the surface located as the lower surface of the first solidelectrolyte layer 1 is referred to as a rear surface Sb of the sensorelement 101. In the determination of the concentration of an unburnedhydrocarbon gas of a measurement gas with the HC sensor 100, apredetermined range starting from a distal end E1 being one end of thesensor element 101, which includes at least the sensing electrode 10, isdisposed in a measurement gas atmosphere, and the other portionincluding a base end E2 being the other end is disposed so as not to bein contact with the measurement gas atmosphere.

The sensing electrode 10 is an electrode for sensing a measurement gas.The sensing electrode 10 is formed as a porous cermet electrode made ofPt containing a predetermined ratio of Au, namely, Pt—Au alloy andzirconia. Such a sensing electrode 10 is provided in a substantiallyrectangular shape in plan view at a position close to the distal end E1,being one end in the longitudinal direction of the sensor element 101,on the front surface Sa of the sensor element 101.

The catalytic activation of the sensing electrode 10 against an unburnedhydrocarbon gas is disabled by preferably setting the composition of aPt—Au alloy being its constituent material. That is, the decompositionreaction of an unburned hydrocarbon gas in the sensing electrode 10 issuppressed. In the HC sensor 100, accordingly, the potential of thesensing electrode 10 selectively varies with respect to (has correlationwith) the unburned hydrocarbon gas, in accordance with itsconcentration. In other words, the sensing electrode 10 is provided soas to have high dependence of potential on concentration for an unburnedhydrocarbon gas while having low dependence of potential onconcentration for components of other measurement gas. This isimplemented by including gold (Au) in the sensing electrode 10 as aconductive component (a noble metal component), in addition to platinum(Pt) being a main constituent.

Specifically, the sensing electrode 10 is formed such that the abundanceratio of Au (Au abundance ratio) in the sensing electrode 10 is 0.3 ormore. Since the sensing electrode 10 is formed in such a manner, in theHC sensor 100, the detection sensitivity becomes higher than the case inwhich the sensing electrode 10 is formed as a cermet electrode of Pt andzirconia, as with the reference electrode 20. Thereby, the HC sensor 100is adapted to be able to detect the unburned hydrocarbon gas at highdetection sensitivity even when an unburned hydrocarbon gas contained inthe exhaust gas G of an oxygen-excessive atmosphere generated in theengine main body 300, as described above, is a detection target.

In this specification, the Au abundance ratio means an area ratio of aportion covered with Au to a portion at which Pt is exposed in thesurface of noble metal particles forming the sensing electrode 10. TheAu abundance ratio is 1 when the area of the portion at which Pt isexposed is equal to the area of the portion covered with Au. In thisspecification, an Au abundance ratio is calculated from a peak intensityof a peak detected for Au and Pt, obtained by X-ray photoelectronspectroscopy (XPS) using a relative sensitivity coefficient method.

For an Au abundance ratio of 0.3 or more, in the sensing electrode 10,Au is concentrated on the surface of noble metal particles forming thesensing electrode 10. In more detail, an Au-rich Pt—Au alloy is formednear the surface of Pt-rich Pt—Au alloy particles. When such a state isachieved, the catalytic activation in the sensing electrode 10 ispreferably disabled, increasing the dependence of the potential of thesensing electrode 10 on the concentration of an unburned hydrocarbongas.

The volume ratio between noble metal components and zirconia in thesensing electrode 10 may be about from 5:5 to 8:2.

For the HC sensor 100 to preferably exhibit its function, the porosityof the sensing electrode 10 is preferably 10% or more and 30% or less,and the thickness of the sensing electrode 10 is preferably 5 μm ormore. In particular, the porosity is more preferably 15% or more and 25%or less, and the thickness is more preferably 25 μm or more and 45 μm orless.

The plane size of the sensing electrode 10 may be appropriately set, andit suffices that, for example, the length in the longitudinal of thesensor element is about 0.2 mm to about 10 mm and the lengthperpendicular to the longitudinal direction is about from 1 mm to 5 mm.

The reference electrode 20 is an electrode substantially rectangular inplan view, which is provided inside the sensor element 101 and serves areference when the concentration of a measurement gas is determined. Thereference electrode 20 is formed as a porous cermet electrode made of Ptand zirconia.

The reference electrode 20 may be formed to have a porosity of 10% ormore and 30% or less and a thickness of 5 μm or more and 15 μm or less.The plane size of the reference electrode 20 may be smaller than that ofthe sensing electrode 10 as illustrated in FIGS. 2A and 2B or may beequal to that of the sensing electrode 10.

The reference gas introduction layer 30 is a layer made of porousalumina, which is provided inside the sensor element 101 to cover thereference electrode 20. The reference gas introduction space 40 is aninternal space provided on the base end E2 side of the sensor element101. Air (oxygen), serving as a reference gas when the concentration ofan unburned hydrocarbon gas is determined, is externally introduced intothe reference gas introduction space 40.

The reference gas introduction space 40 and the reference gasintroduction layer 30 are in communication with each other, andaccordingly, in the use of the HC sensor 100, the surroundings of thereference electrode 20 are always filled with air (oxygen) through thereference gas introduction space 40 and the reference gas introductionlayer 30. During the use of the HC sensor 100, therefore, the referenceelectrode 20 always has a constant potential.

The reference gas introduction space 40 and the reference gasintroduction layer 30 are provided so as not to come into contact with ameasurement gas owing to their surrounding solid electrolytes. Thisprevents the reference electrode 20 from coming into contact with themeasurement gas even if the sensing electrode 10 is exposed to themeasurement gas.

In the case illustrated in FIGS. 2A and 2B, the reference gasintroduction space 40 is provided in such a manner that part of thefifth solid electrolyte layer 5 is in communication with the outside onthe base end E2 side of the sensor element 101. The reference gasintroduction layer 30 is provided so as to extend in the longitudinaldirection of the sensor element 101 between the fifth solid electrolytelayer 5 and the sixth solid electrolyte layer 6. The reference electrode20 is provided at a position below the center of gravity of the sensorelement 101 in FIGS. 2A and 2B.

The surface protective layer 50 is a porous layer made of alumina, whichis provided so as to cover at least the sensing electrode 10 on thefront surface Sa of the sensor element 101. The surface protective layer50 is provided as an electrode protective layer that prevents or reducesthe degradation of the sensing electrode 10 due to continuous exposureto a measurement gas during the use of the HC sensor 100. In the caseillustrated in FIGS. 2A and 2B, the surface protective layer 50 isprovided so as to cover not only the sensing electrode 10 but alsosubstantially all the portion of the front surface Sa of the sensorelement 101 except for a predetermined range starting from the distalend E1.

As shown in FIG. 2B, the HC sensor 100 is equipped with a potentiometer60 capable of measuring a potential difference between the sensingelectrode 10 and the reference electrode 20. Although FIG. 2Bschematically shows wiring between the potentiometer 60 and the sensingelectrode 10 as well as the reference electrode 20, in an actual sensorelement 101, connection terminals (not shown) are providedcorrespondingly to the electrodes on the front surface Sa or the rearsurface Sb on the base end E2 side, and wiring patterns (not shown),which connect the electrodes and their corresponding connectionterminals, are formed on the front surface Sa and in the element. Thesensing electrode 10 and the reference electrode 20 are electricallyconnected with the potentiometer 60 via the wiring patterns and theconnection terminals. In the preferred embodiment, the potentialdifference between the sensing electrode 10 and the reference electrode20, measured by the potentiometer 60, serves as a HC detection signalsg11. In addition, such a potential difference is also referred to as aHC sensor output.

The sensor element 101 further includes a heater part 70, which performstemperature control of heating the sensor element 101 and maintainingthe temperature of the sensor element 101, to enhance the oxygen ionconductivity of the solid electrolyte. The heater part 70 includes aheater electrode 71, a heater 72, a through-hole 73, a heater insulatinglayer 74, and a pressure diffusion hole 75.

The heater electrode 71 is an electrode formed so as to come intocontact with the rear surface Sb of the sensor element 101 (the lowersurface of the first solid electrolyte layer 1 in FIGS. 2A and 2B).Connecting the heater electrode 71 with an external power source (notshown) enables power feeding from the outside to the heater part 70.

The heater 72 is an electric resistor provided inside the sensor element101. The heater 72, which is connected with the heater electrode 71through the through-hole 73, generates heat by being fed power from theoutside via the heater electrode 71 to heat the solid electrolytesforming the sensor element 101 and maintain their temperature.

In the case illustrated in FIGS. 2A and 2B, the heater 72 is buriedwhile being vertically sandwiched between the second solid electrolytelayer 2 and the third solid electrolyte layer 3 so as to range from thebase end E2 to the position below the sensing electrode 10 near thedistal end E1. This enables the adjustment of the entire sensor element101 to the temperature at which the solid electrolytes are activated.

The heater insulating layer 74 is an insulating layer made of aninsulator such as alumina on the upper and lower surfaces of the heater72. The heater insulating layer 74 is formed for the electricalinsulation between the second solid electrolyte layer 2 and the heater72 and for the electrical insulation between the third solid electrolytelayer 3 and the heater 72.

The pressure diffusion hole 75 is a part provided to penetrate the thirdsolid electrolyte layer 3 and to be in communication with the referencegas introduction space 40, which is formed to mitigate an internalpressure rise associated with a temperature rise in the heaterinsulating layer 74.

When the concentration of an unburned hydrocarbon gas in the exhaust gasG from the engine main body 300 being a measurement gas is determined byuse of the HC sensor 100 having the configuration described above, air(oxygen) is supplied to the reference gas introduction space 40, withthe sensor element 101 in only a predetermined range, which starts fromthe distal end E1 and includes at least the sensing electrode 10, beingdisposed in an exhaust pipe 500 of an engine system 1000, and with thesensor element 101 on the base end E2 side being disposed apart from thespace, as described above. The sensor element 101 is heated at anappropriate temperature of 300° C. to 800° C., preferably 400° C. to700° C., and more preferably 400° C. to 600° C. by the heater 72.

In such a state, a potential difference occurs between the sensingelectrode 10 exposed to the measurement gas (exhaust gas G) and thereference electrode 20 arranged in the atmosphere. As described above,however, the potential of the reference electrode 20 disposed under theair (having a constant oxygen concentration) atmosphere is maintained ata constant potential, whereas the potential of the sensing electrode 10selectively has concentration dependence on the unburned hydrocarbon gasin the measurement gas (exhaust gas G), and therefore their potentialdifference (HC sensor output) substantially has a value corresponding tothe concentration of the measurement gas present around the sensingelectrode 10. Therefore, a certain functional relation (referred to assensitivity characteristics) holds between the concentration of anunburned hydrocarbon gas and sensor output.

In the oxidation catalyst diagnosis system DS1, under the premise thatthe functional relation holds, the deterioration diagnosis of theoxidation catalyst 600 is performed by use of the HC detection signalsg11 which is the output from the HC sensor 100.

That is, sensitivity characteristics are experimentally identified bymeasuring sensor output in advance with a plurality of different mixedgases, each of which has a known unburned hydrocarbon gas concentration,used as measurement gases, and are stored in the electronic controldevice 200. The concentration of an unburned hydrocarbon gas on adownstream side of the oxidation catalyst 600 can be determined almostin real time by converting a HC sensor output, which varies from momentto moment in accordance with the concentration of an unburnedhydrocarbon gas of a measurement gas, into the concentration of anunburned hydrocarbon gas on the basis of the sensitivity characteristicsby the electronic control device 200. In the electronic control device200, on the basis of the concentration of an unburned hydrocarbon gas,the degree of the deterioration of the oxidation catalyst 600 isdetermined.

Alternatively, it may be employed that the conversion to an unburnedhydrocarbon gas concentration equivalent basis is omitted and a HCsensor output value (potential difference value) itself which is givenas a HC detection signal sg11 is used for the deterioration diagnosis.

<Characteristic of Oxidation Catalyst and Basic Concept of DeteriorationDiagnosis>

Next, characteristics of the oxidation catalyst 600 being a target ofthe deterioration diagnosis by the oxidation catalyst diagnosis systemDS1 of the preferred embodiment, and a basic concept of thedeterioration diagnosis will be described.

First, the characteristics of the oxidation catalyst 600 will bedescribed. FIG. 3 is a view schematically showing a relation (conversionrate profile) between a temperature (catalyst temperature) and aconversion rate of the oxidation catalyst 600. FIG. 3 schematicallyshows the conversion rate profile Pf of the oxidation catalyst 600(fresh article or also referred to just as fresh) in the Initial use(unused or just-started use) and the conversion rate profile Pa of theoxidation catalyst 600 (aged article or also referred to just as aged)used for a certain period of time.

The conversion rate is a value serving as an index of catalytic abilityin the oxidation catalyst 600, and when the concentration of theunburned hydrocarbon gas in the vicinity on an upstream side of theoxidation catalyst 600 is referred to as a concentration Nu of anunburned hydrocarbon gas on an upstream side and the concentration ofthe unburned hydrocarbon gas in the vicinity on a downstream side isreferred to as a concentration Nl of an unburned hydrocarbon gas on andownstream side, the conversion rate is defined by the following formula(1).

Conversion rate (%)=100×(Nu−Nl)/Nu  (Formula 1)

That is, the conversion rate represents a ratio of an unburnedhydrocarbon gas not discharged from the oxidation catalyst 600 to adownstream side to an unburned hydrocarbon gas flown into the oxidationcatalyst 600 from an upstream side. An oxidation catalyst 600 having ahigher conversion rate comes to have higher catalytic ability.

More specifically, in a temperature range (adsorption region in FIG. 3)below a certain temperature T1 (about 150° C.), the oxidation catalyst600 has a function (adsorptive capacity) of exclusively adsorbing anunburned hydrocarbon gas, and in a temperature range (oxidation regionin FIG. 3) above a certain temperature T2 (commonly 150° C. to 200° C.),the oxidation catalyst 600 suitably exerts a capacity (oxidizingability) of oxidizing an unburned hydrocarbon gas being an inherentfunction. In the temperature range between the temperature T1 and thetemperature T2 (intermediate region in FIG. 3), the higher thetemperature is, the lower the adsorptive capacity becomes and the higherthe oxidizing ability becomes. Thus, the conversion rate is a valuerepresenting a ratio of an unburned hydrocarbon gas adsorbed or oxidizedin the oxidation catalyst 600 to an unburned hydrocarbon gas flown froman upstream side into the oxidation catalyst 600.

As shown in FIG. 3, a conversion rate profile Pf of the fresh article isusually the highest (approximately about 90%) in the oxidation regionand the conversion rate in the adsorption region is lower than that inthe oxidation region. Furthermore, the conversion rate becomes minimumat an upper limit temperature T1 (about 150° C.) of the adsorptionregion, and the conversion rate tends to increase as a temperatureincreases in the intermediate region.

However, a temperature of the oxidation catalyst 600 can vary from hourto hour depending on a temperature of the exhaust gas G (exhaust gastemperature) which is discharged from the engine main body 300 and flowsinto through the exhaust pipe 500, and therefore an actual conversionrate varies from hour to hour.

The oxidation catalyst 600 having a high conversion rate at the stage ofa fresh article is deteriorated due to continued use. That is, theconversion rate of the oxidation catalyst 600 is reduced regardless of atemperature as the fresh article becomes the aged article due to thecontinued use. A magnitude relation between the adsorption region andthe oxidation region in a conversion rate profile Pa of the aged articleis mainly similar to that of the Pf of the fresh article; however theconversion rate of the aged article is smaller than that of the fresharticle at the same temperature.

As a result of the use thus continued of the oxidation catalyst 600,once the conversion rate is reduced below a predetermined level, theoxidation catalyst 600 cannot exert an inherent desired function. Forexample, in the case that the engine system 1000 is mounted on anautomobile, a problem such that the automobile does not satisfyenvironmental standards arises. The oxidation catalyst diagnosis systemDS1 of the preferred embodiment enables to replace the aged article ofthe oxidation catalyst 600 in the engine system 1000 with the fresharticle timely by determining whether the deterioration occurs to acontroversial extent (to the extent requiring replacement, etc.) in theoxidation catalyst 600 being the aged article.

Conceptually, data (threshold data) corresponding to a threshold profileT in which the threshold value of the conversion rate, as described inFIG. 3, is set every temperature, is previously prepared and stored inthe electronic control device 200 constituting the oxidation catalystdiagnosis system DS1. And then, a temperature and a conversion rate ofthe oxidation catalyst 600 being a diagnosis target are determined. Inthe case that the obtained conversion rate is smaller than the thresholdvalue at the temperature, it is determined that the oxidation catalyst600 is deteriorated.

However, while it is necessary to determine the concentration of theunburned hydrocarbon gas on both of upstream and downstream sides of theoxidation catalyst 600 in order to actually calculate the conversionrate, the oxidation catalyst diagnosis system DS1 of the preferredembodiment is not provided with a HC sensor on an upstream side of theoxidation catalyst 600. In the preferred embodiment, using the fact thatthe concentration Nu of an unburned hydrocarbon gas on an upstream sideof the oxidation catalyst 600 lies in a certain range, the oxidationcatalyst diagnosis system DS1 is adapted to diagnose the degree ofdeterioration of the oxidation catalyst 600 based on the concentrationNl of an unburned hydrocarbon gas on a downstream side determined basedon the output value (HC detection signal sg11) in the HC sensor 100provided downstream of the oxidation catalyst 600, the temperature ofthe oxidation catalyst 600 identified by an exhaust gas temperaturedetection signal sg12 from a temperature sensor 110 provided upstream ofthe oxidation catalyst 600, and threshold data of the concentration Nlof an unburned hydrocarbon gas on a downstream side which is previouslydefined according to the temperature of the catalyst and stored in thestorage portion of the electronic control device 200. That is, theoxidation catalyst diagnosis system DS1 is adapted to performdeterioration diagnosis by using the concentration of an unburnedhydrocarbon gas in the exhaust gas G undergoing the oxidation catalyst600 as a direct diagnosis target. A specific content of the diagnosiswill be described later.

FIG. 4 is a view showing output values of a HC sensor 100 in flowingactual exhaust gases (containing HC, CO and O₂) as the test gases havingthe approximately same unburned hydrocarbon gas concentrations from anupstream side, on both cases where an oxidation catalyst 600 being afresh article is used and an oxidation catalyst 600 being an agedarticle is used in the diesel engine system 1000. In addition, thetemperature of the oxidation catalyst 600 is set to 200° C. As shown inFIG. 4, the output is almost close to 0 mV in the fresh article, whereasthe output of 200 mV is obtained in the aged article. This result meansthat almost all of the unburned hydrocarbon gas is oxidized in the fresharticle, whereas the unburned hydrocarbon gas is not completelyoxidized, and the remainder is discharged as it is in the aged article.

Further, FIG. 5 is a view showing the results at the time when in thedetection by the HC sensor 100 shown in FIG. 4, gas analysis isperformed using an gas analyzer at the downstream of the HC sensor 100.In addition, “THC” of a vertical axis on a left side in FIG. 5 means atotal of hydrocarbon gases (typical hydrocarbon gas) excluding CO (sameapplies to following drawings). It is confirmed from FIG. 5 thatmeasured concentration values of THC and CO of the aged article arehigher than those of the fresh article by ΔHC and ΔCO, respectively.Since the concentration Nu of an unburned hydrocarbon gas on an upstreamside is constant, the results shown in FIG. 4 and FIG. 5 suggest thatwhen the oxidation catalyst diagnosis system DS1 of the preferredembodiment is used, in the aged article, the occurrence of thedeterioration of the conversion rate in the oxidation catalyst 600 canbe detected based on the results of direct measurement of unburnedhydrocarbon gas in the exhaust gas by the HC sensor 100. A specificdiagnosis procedure will be described later.

Strictly speaking, since an amount of the unburned hydrocarbon gasoxidized in the fresh article is larger than that in the aged article,strictly, an oxygen concentration of the fresh article is expected to behigher than that of the aged article; however, there is littledifference in oxygen concentration between the fresh article and theaged article in FIG. 5. The reason for this is that the concentration(ppm order) of oxygen generated by oxidation is low enough compared withthe concentration (% order) of oxygen inherently present in a test gasflown into from an upstream side.

Further, a value of the conversion rate profile Pa of the aged article,shown in FIG. 3, is smaller than a value of the conversion rate profilePf by an approximately constant value at all temperatures; however, thisis just an exemplification. FIGS. 6A and 6B are each a viewschematically showing another example of a conversion rate profile Pa.That is, as for the conversion rate profile Pa of the aged article,there may be cases where the degree of deterioration in the adsorptionregion is larger than that in the oxidation region as shown in FIG. 6A,and in contrast, there may be cases where the degree of deterioration inthe oxidation region is larger than that in the adsorption region asshown in FIG. 6B. Accordingly, in the case of FIG. 6A, it is preferredto timely detect the deterioration occurring in a temperature region ofthe temperature T1 or less, and in the case of FIG. 6B, it is preferredto timely detect the deterioration occurring in a temperature region ofthe temperature T2 or more.

<Comparison with Other Technique>

Next, as for a technique of directly measuring an unburned hydrocarbongas in the exhaust gas G by the HC sensor 100, which is performed in theoxidation catalyst diagnosis system DS1 of the preferred embodiment, theadvantageous effect over other oxidation catalyst diagnosis techniqueswill be described.

FIG. 7 is a view showing changes in an output value in the HC sensor 100at the time when an extremely small amount of fuel is injected for ashort time from a fuel injection valve 301 in the middle of an operationunder a certain condition of an engine system 1000 provided with an agedarticle as an oxidation catalyst 600, as well as changes in ahydrocarbon gas concentration in a gas analyzer provided in the vicinityof the HC sensor 100. In addition, “ppmc” of the unit of a vertical axison a right side in FIG. 7 indicates to represent a hydrocarbon gasconcentration on the carbon atom equivalent basis by the unit of ppm.

When such an extremely small amount of fuel injection is executed inaddition to the exhaust gas G steadily discharged in association with anoperation of the engine main body 300, injected fuel vaporized inside ofthe engine main body 300 is discharged to the exhaust pipe 500 in asuperimposed manner, and therefore an unburned hydrocarbon gas is sentto the oxidation catalyst 600 instantaneously at a concentration higherthan a steady state. In addition, more specifically, injection of thefuel was executed under the conditions of a delay angle of 88°, aninjection amount of 1 mg/stroke and an injection time of 6 seconds at apost-injection in the engine system 1000 in which an engine speed is2500 rpm and torque is 20 Nm. In addition, the temperature of theoxidation catalyst 600 identified from an output value of thetemperature sensor 110 was 200° C.

From FIG. 7, a peak corresponding to fuel injection is recognized oneach of the output profile of the HC sensor 100 and the output profileof the gas analyzer, and it is understood that both profile shapessubstantially agree with a time axis. This indicates that the oxidationcatalyst diagnosis system DS1 of the preferred embodiment has excellentresponsiveness to the variation of the concentration of an unburnedhydrocarbon gas and can detect the unburned hydrocarbon gas with highaccuracy and in real time, and hence means that the system can promptlyperform the deterioration diagnosis of the oxidation catalyst 600.

On the other hand, FIG. 8 and FIG. 9, shown for comparison, are eachview showing an appearance of changes in output value which are obtainedunder the same conditions as in the results shown in FIG. 7 and requiredfor applying a ΔT method and a Δλ method.

The results shown in FIG. 8 was obtained by measuring a temperature onan upstream side of the oxidation catalyst 600 by the temperature sensor110, and measuring a temperature on the downstream side by anothertemperature sensor provided for evaluation on a downstream side of theoxidation catalyst 600. FIG. 8 shows that a temperature rise occurringin response to fuel injection on both of upstream/downstream sides isdetected, but peak positions are inevitably deviated due to a differencein installation position and the peak of downstream is broad withrespect to a time axis. Further, there is little difference intemperature between peaks. In addition, not shown in drawings, adifference in temperature value is finally recognized between two peakswhen the fuel injection amount is increased.

It can be said from such a result that the diagnosis technique performedbased on the oxidation catalyst diagnosis system DS1 of the preferredembodiment is superior in measurement accuracy and promptness ofdiagnosis to the ΔT method.

Further, the results shown in FIG. 9 was obtained by providing λ sensorfor evaluation on both of upstream/downstream sides of the oxidationcatalyst 600, providing a gas analyzer used in measurement shown in FIG.7 on both of upstream/downstream sides of the oxidation catalyst 600,and measuring by the λ sensor and the gas analyzer. Comparing outputprofiles of the gas analyzers on upstream/downstream sides of fourprofiles shown in FIG. 9, since a reduction of an oxygen concentrationassociated with fuel injection on the downstream is larger than that onthe upstream, it is confirmed that oxygen is reduced by the oxidation ofthe unburned hydrocarbon gas in the oxidation catalyst 600. However,comparing the output profiles of the λ sensor on upstream/downstreamsides, degrees of reduction of λ values associated with fuel injectionare not different. Since λ sensor measures a concentration of remainingoxygen after combustion of the unburned hydrocarbon and oxygen, adifference in output between the upstream λ sensor and the downstream λsensor is not generated in principle.

This result indicates that a reduction of oxygen by the oxidation of theunburned hydrocarbon gas in the oxidation catalyst 600 is not capturedby two λ sensors. This means in short that the oxidation catalystdiagnosis system DS1 of the preferred embodiment can realize thediagnosis of the degree of deterioration of the oxidation catalyst 600which cannot be performed by the Δλ method.

<Specific Diagnosis Technique-Passive OBD and Active OBD>

Subsequently, a specific technique of diagnosing the degree ofdeterioration of the oxidation catalyst 600 which is performed in theoxidation catalyst diagnosis system DS1 of the preferred embodiment,will be described. A diagnosis technique performed in the preferredembodiment is broadly divided into Passive OBD and Active OBD,appropriately used separately according to the purpose, or used incombination thereof.

In the preferred embodiment, Passive OBD, schematically describing, is atechnique of performing diagnosis by directly using a state of anordinary operation under which the engine main body 300 continuouslydischarges the exhaust gas G through the exhaust pipe 500. Thus, anunburned hydrocarbon gas to be used for diagnosis in Passive OBD islimited to the unburned hydrocarbon gas contained in the exhaust gas G.

Passive OBD can be performed at any timing as long as the engine mainbody 300 is under an ordinary operation state. Or, it may be employedthat the oxidation catalyst diagnosis system DS1 automatically performsPassive OBD intermittently or continuously, even though a particularinstruction of execution is not given. However, since the concentrationof an unburned hydrocarbon gas being a detection target by the HC sensor100 does not exceed a concentration in the exhaust gas G discharged fromthe engine main body 300, particularly, in the oxidation region in whichthe conversion rate is relatively high even in the case of the agedarticle, its detection amount tends to decrease. In some cases, thispoint may have an effect on diagnosis accuracy.

On the other hand, in the preferred embodiment, Active OBD,schematically describing, is a technique in which an extremely smallamount of fuel is intentionally injected for a short time during anoperation of the engine main body 300 to generate a hydrocarbon gas fordiagnosis, and diagnosis is performed with an atmosphere including thegenerated hydrocarbon gas targeted for diagnosis. That is, a hydrocarbongas atmosphere to be used for diagnosis in Active OBD is formed bysuperimposing such a hydrocarbon gas generated for diagnosis on anunburned hydrocarbon gas contained in an ordinary exhaust gas G.However, in the following description, for convenience sake, ahydrocarbon gas generated for diagnosis is referred to as an unburnedhydrocarbon gas. When performing Active OBD, even if the oxidationcatalyst 600 is a fresh article with high conversion rate, since adetection amount of the unburned hydrocarbon gas in the HC sensor 100 isincreased compared with Passive OBD, it becomes possible to perform thediagnosis more certainly.

Although Active OBD seems disadvantageous from the viewpoint of securingfuel economy in the engine system 1000 since it intentionally performsfuel injection, as described later, the total amount of fuel injectionperformed in the oxidation catalyst diagnosis system DS1 of thepreferred embodiment, is small enough compared with fuel injectionperformed for other purpose in the engine system 1000 or fuel injectionin performing other diagnosis technique, and an impact on fuel economyremains in the minimum.

<Example of Procedure of Passive OBD>

FIG. 10 is a view showing an example of a procedure of diagnosis basedon Passive OBD. The diagnosis by Passive OBD starts with checking atemperature of the exhaust gas G (exhaust gas temperature) in thevicinity on an upstream side of the oxidation catalyst 600 by thetemperature sensor 110 (step S1). More specifically, the electroniccontrol device 200 acquires exhaust gas temperature detection signalssg12 sent from the temperature sensor 110 and thereby the exhaust gastemperature is identified. Such an exhaust gas temperature is regardedas a temperature of the oxidation catalyst 600 at the point in time.

Subsequently, the electronic control device 200 calls the thresholdvalue of the concentration of an unburned hydrocarbon gas for PassiveOBD corresponding to the exhaust gas temperature from the threshold datafor Passive OBD previously stored in the storage portion (step S2). Thethreshold value is previously set in all temperatures within a possibletemperature range of the oxidation catalyst 600 (approximately −40° C.to 1000° C.). Since there is not a particular restriction on how to givethe threshold value, the threshold value may be given as a continuousfunction of a temperature of the oxidation catalyst 600 (exhaust gastemperature) or given as a fixed value for every temperature range.

Then, in the HC sensor 100, an unburned hydrocarbon gas concentration Nlon a downstream side of the oxidation catalyst 600 is measured (stepS3). More specifically, the exhaust gas G whose temperature has beenmeasured by the temperature sensor 110 reaches the oxidation catalyst600, adsorption or oxidation of an unburned hydrocarbon gas takes placeinside the oxidation catalyst 600, and the rest of the unburnedhydrocarbon gas is discharged to the downstream side, and at thistiming, the electronic control device 200 acquires, as a HC detectionsignal sg11, a potential difference generated between the sensingelectrode 10 and the reference electrode 20 (HC sensor output) in the HCsensor 100, and the unburned hydrocarbon gas concentration Nl on adownstream side is calculated based on a value of the potentialdifference and a sensitivity characteristic of the HC sensor 100 whichhas been identified in advance.

Check of the exhaust gas temperature by the temperature sensor 110 andmeasurement of the concentration Nl of an unburned hydrocarbon gas on adownstream side by the HC sensor 100 may be performed simultaneously.

After measurement of the concentration Nl of an unburned hydrocarbon gason a downstream side and call of the threshold value, the electroniccontrol device 200 compares the measured value to the threshold value(step S4), and when the former is larger than the latter (YES in thestep S4), the electronic control device 200 determines thatdeterioration occurs to a controversial extent (requiring replacement,etc.) in the oxidation catalyst 600 (to be NG) (step S5), and when thelatter is larger than the former (NO in the step S4), the electroniccontrol device 200 determines that such deterioration does not occur (tobe OK) (step S6).

When diagnosis is repeated after the diagnosis regardless of whetherdetermined to be NG or to be OK (YES in the step S7), this process isrepeated again from checking an exhaust gas temperature by thetemperature sensor 110. If not so, the diagnosis is ended as it is (NOin the step S7).

<Detail of Diagnosis by Passive OBD>

The reason why the degree of deterioration of the oxidation catalyst 600can be diagnosed by the procedure shown in FIG. 10, particularly, thereason why the deterioration can be diagnosed by comparing the measuredvalue of the concentration Nl of an unburned hydrocarbon gas on adownstream side to the threshold value will be described based on FIG.11 and FIG. 12.

First, FIG. 11 is a view showing a relation between the concentration Nuof an unburned hydrocarbon gas on an upstream side and the concentrationNl of an unburned hydrocarbon gas on a downstream side in severalconversion rates in the case where a temperature of an oxidationcatalyst 600 is in an oxidation region in which a temperature is higherthan T2 (hereinafter, the temperature of the oxidation catalyst 600 inthis case is denoted by Ta). For example, Ta is 200° C.

On the other hand, FIG. 12 is a view showing a relation between theconcentration Nu of an unburned hydrocarbon gas on an upstream side andthe concentration Nl of an unburned hydrocarbon gas on a downstream sidein several conversion rates in the case where a temperature of anoxidation catalyst 600 is in an adsorption region in which a temperatureis T1 or lower (hereinafter, the temperature of the oxidation catalyst600 in this case is denoted by Tb). For example, Tb is 150° C.

Since the conversion rate is defined by the above-mentioned Formula 1,when the horizontal axis corresponds to the concentration Nu of anunburned hydrocarbon gas on an upstream side and the vertical axiscorresponds to the concentration Nl of an unburned hydrocarbon gas on adownstream side like FIG. 11 and FIG. 12, the conversion rate isrepresented by a function of a slope of a straight line. Specifically,the slope of a straight line decreases as the conversion rate of theoxidation catalyst 600 increases. In addition, in FIG. 11 and FIG. 12,the concentration Nu of an unburned hydrocarbon gas on an upstream sideis represented by the term “upstream HC concentration” and theconcentration Nl of an unburned hydrocarbon gas on a downstream side isrepresented by the term “downstream HC concentration” (the same appliesto FIG. 14 and FIG. 15).

In addition, in the preferred embodiment, a range of the conversion ratein which it is provided that controversial deterioration does not occur(acceptable) in the oxidation catalyst 600, is referred to as anallowable conversion rate range. The allowable conversion rate range maybe optionally set in consideration of a state of the oxidation catalyst600, whereas an upper limit thereof is usually 100% and therefore only alower limit is optionally set.

In FIG. 11 showing an example at the temperature Ta, a relation betweenthe concentration Nu of an unburned hydrocarbon gas on an upstream sideand the concentration Nl of an unburned hydrocarbon gas on a downstreamside is indicated by a thin solid line, a broken line and a thick solidline for the conversion rates of 90%, 70% and 20%, respectively. Inaddition, in the case of the temperature Ta, the oxidation catalysthaving a conversion rate of 90% is assumed as a fresh article. Forexample, in the case of a fresh article having a conversion rate of 90%,when the concentration Nu of an unburned hydrocarbon gas on an upstreamside of the oxidation catalyst 600 is 1000 ppm, the concentration Nl ofan unburned hydrocarbon gas on a downstream side of the oxidationcatalyst 600 is 100 ppm.

On the other hand, in FIG. 12 showing an example at temperature Tb, arelation between the concentration Nu of an unburned hydrocarbon gas onan upstream side and the concentration Nl of an unburned hydrocarbon gason a downstream side is indicated by a thin solid line, a broken lineand a thick solid line for the conversion rates of 50%, 30% and 10%,respectively. In addition, when the temperature is Tb, the oxidationcatalyst having a conversion rate of 50% is assumed as a fresh article.Incidentally, as described above, the conversion rate in the adsorptionregion is usually smaller than the conversion rate in the oxidationregion.

Further, as described above, in the preferred embodiment, it ispreviously found empirically that the concentration Nu of an unburnedhydrocarbon gas on an upstream side varies constantly depending onoperating conditions of the engine main body 300; however, theconcentration Nu varies within a predetermined concentration range. InFIG. 11 and FIG. 12, a range of at least 400 ppm and at most 1000 ppm isassumed to be the variation range R1 of the concentration Nu of anunburned hydrocarbon gas on an upstream side (hereinafter, referred toas a variation range of upstream HC concentration). This means that,although a specific value of the concentration Nu of an unburnedhydrocarbon gas on an upstream side in performing the diagnosis byPassive OBD is not identified in the preferred embodiment sincemeasurement is not performed, the concentration value is surely regardedas a value within a range of the variation range R1 of upstream HCconcentration.

Here, when the conversion rate is 70% or more in the case of thetemperature Ta shown in FIG. 11, it is provided that controversialdeterioration does not occur in the oxidation catalyst 600. In thiscase, a range of 70% or more is an allowable conversion rate range. Ameasurement value of the concentration Nl of an unburned hydrocarbon gason a downstream side of the oxidation catalyst 600 satisfying such anallowable conversion rate range is 300 ppm or less. This is because,when the conversion rate is 70% or more, the slope of the straight linecorresponding to Formula 1 in FIG. 11 is always smaller than the slopeof the straight line of the conversion rate of 70%, which passes througha point (1000, 300).

Namely, this means that, if the threshold value to be compared with ameasurement value of the concentration Nl of an unburned hydrocarbon gason a downstream side by the HC sensor 100 is set to 300 ppm, all of theoxidation catalysts 600 in which an actual conversion rate lies withinan allowable conversion rate range of 70% or more, are certainlydetermined to be OK in Passive OBD.

In addition, in the case of such a temperature Ta, if the thresholdvalue is set to 300 ppm, a value of the concentration Nl of an unburnedhydrocarbon gas on a downstream side becomes larger than 300 ppm in theoxidation catalysts 600 in which an actual conversion rate is less than20% no matter which value in the variation range R1 of upstream HCconcentration the concentration Nu of an unburned hydrocarbon gas on anupstream side is. Namely, this means that when the threshold value isset to 300 ppm, all of the oxidation catalysts 600 in which an actualconversion rate is less than 20%, are certainly determined to be NG inPassive OBD.

On the other hand, in the case of the temperature Tb shown in FIG. 12, arange of the conversion rate of 30% or more is defined as an allowableconversion rate range considering that a temperature of the oxidationcatalyst 600 lies in the adsorption region, and a threshold value is setto 700 ppm higher than the case of the temperature Ta. In such a case,controversial deterioration does not occur in the oxidation catalyst 600as long as the conversion rate satisfies the allowable conversion raterange of 30% or more, and the oxidation catalyst 600 is certainlydetermined to be OK.

However, in the case of such a temperature Tb, the oxidation catalyst600 can be determined to be OK even if the conversion rate of theoxidation catalyst 600 is 0%. This is because, when the conversion rateis 0%, possible values of the concentration Nl of an unburnedhydrocarbon gas on a downstream side is at least 400 ppm and at most1000 ppm similar to those of the concentration Nu of an unburnedhydrocarbon gas on an upstream side, and therefore the possible valuemay become smaller than 700 ppm of the threshold value. Thus, such acondition that the oxidation catalyst 600 is certainly determined to beNG does not occur in contrast to the case of the temperature Ta.

In contrast to these, for example, when an actual conversion rate of theoxidation catalyst 600 in the case of the temperature Ta is at least 20%and not more than 70% like a straight line L indicated by a chaindouble-dashed line in FIG. 11, whether the oxidation catalyst 600 isdetermined to be NG or to be OK in Passive OBD depends on an actualconcentration Nu of an unburned hydrocarbon gas on an upstream side induring diagnosis which is not known as a value. More specifically, thecase in which a value of the concentration Nl of an unburned hydrocarbongas on a downstream side is 300 ppm becomes a boundary, if a value ofthe concentration Nu of an unburned hydrocarbon gas on an upstream sideis close to 400 ppm, the catalyst is determined to be OK since the valueof the concentration Nl of an unburned hydrocarbon gas on a downstreamside is less than 300 ppm (falls within a range of less than 300 ppm inthe straight line L), and if a value of the concentration Nu of anunburned hydrocarbon gas on an upstream side is close to 1000 ppm, thecatalyst is determined to be NG since the value of the concentration Nlof an unburned hydrocarbon gas on a downstream side is 300 ppm or more(falls within a range of 300 ppm or more in the straight line L).

Similarly, when the conversion rate is at least 0% and not more than 30%in the case of the temperature Tb, whether the oxidation catalyst 600 isdetermined to be NG or to be OK in Passive OBD depends on a value of anactual concentration Nu of an unburned hydrocarbon gas on an upstreamside in during diagnosis. More specifically, the case in which a valueof the concentration Nl of an unburned hydrocarbon gas on a downstreamside is 700 ppm becomes a boundary, if a value of the concentration Nuof an unburned hydrocarbon gas on an upstream side is close to 400 ppm,the catalyst is determined to be OK since the value of the concentrationNl of an unburned hydrocarbon gas on a downstream side is less than 700ppm (falls within a range of less than 700 ppm in the straight line L),and if a value of the concentration Nu of an unburned hydrocarbon gas onan upstream side is close to 1000 ppm, the catalyst is determined to beNG since the value of the concentration Nl of an unburned hydrocarbongas on a downstream side is 700 ppm or more (falls within a range of 700ppm or more in the straight line L).

These mean that even though the actual conversion rate of the oxidationcatalyst 600 is smaller than a lower limit value of the allowableconversion rate range (70% for the temperature Ta and 30% for thetemperature Tb) which is a reference in setting the threshold valuerelated to the concentration Nl of an unburned hydrocarbon gas on adownstream side, the oxidation catalyst 600 can be determined to be OK.However, although the value of the concentration Nu of an unburnedhydrocarbon gas on an upstream side arbitrarily varies within thevariation range R1 of upstream HC concentration, the smaller the actualconversion rate is, the larger a concentration range in which thecatalyst is determined to be NG in the variation range R1 of upstream HCconcentration, and therefore a probability of being determined to be NGis estimated to be large. That is, it is thought that the frequency ofbeing faultily determined to be OK is high when the actual conversionrate is close to the lower limit value of the allowable conversion raterange being a reference value in setting a threshold value. Thissuggests that with respect to the threshold value in performing thediagnosis by Passive OBD, although the threshold value is set accordingto the allowable conversion rate range which can be arbitrarily defined,if the threshold value of the concentration Nl of an unburnedhydrocarbon gas on a downstream side is set a little less (that is, theconversion rate giving a threshold value is set a little more), that isto say, the threshold value is set to a safety side, a practical problemis small even though some wrong diagnosis is generated.

Or, even though the catalyst happens to be faultily determined to be OKby one diagnosis, if the diagnosis system is configured to diagnoserepeatedly as required, it is thought, for the oxidation catalyst 600 inwhich an actual conversion rate becomes smaller due to advancingdeterioration, that a frequency (probability) of being determined to beNG is larger. Thus, it is conceivable to employ a provision in whichpractical detection accuracy is enhanced by selecting to performrepetitive diagnosis in the procedure of FIG. 10, and configuring thesystem to accumulate the result of diagnosis for every diagnosis in astorage portion (not shown) of the electronic control device 200, and todiagnose the degree of deterioration of the oxidation catalyst 600 basedon the accumulated results. For example, it may be employed that when ofnumber of times of predetermined diagnosis, the frequency of beingdetermined to be NG is a certain reference number or less, thecontroversial deterioration is determined not to occur in the oxidationcatalyst 600, and when the number of times exceeds a certain referencenumber, the controversial deterioration is determined to occur in theoxidation catalyst 600. In addition, when the repetitive diagnosis isperformed, since the temperature of the oxidation catalyst 600 can varydiagnosis-to-diagnosis, the used threshold value can differdiagnosis-to-diagnosis.

Or, if the diagnosis is intermittently (with time) performed from thestage of Fresh article, the occurrence of deterioration of the oxidationcatalyst 600 can be captured. This is because the article which isdetermined in succession to be OK at the first may eventually bedetermined to be NG even though the timing of NG may be varied a little.

These things means that it is possible to diagnose the degree ofdeterioration of the oxidation catalyst 600 with practicallysatisfactory accuracy as compared to required diagnosis accuracy even inthe manner that the threshold value is set for only the concentration Nlof an unburned hydrocarbon gas on a downstream side like the diagnosisby Passive OBD performed in the preferred embodiment.

<Example of Procedure of Active OBD>

FIG. 13 is a view showing an example of a procedure of diagnosis byActive OBD. The diagnosis by Active OBD starts with checking atemperature of the exhaust gas G (exhaust gas temperature) in thevicinity on an upstream side of the oxidation catalyst 600 by thetemperature sensor 110, as with Passive OBD (step S11). Morespecifically, the electronic control device 200 acquires exhaust gastemperature detection signals sg12 sent from the temperature sensor 110and thereby the exhaust gas temperature is identified. Such an exhaustgas temperature is regarded as a temperature of the oxidation catalyst600 at the point in time.

Subsequently, the electronic control device 200 calls the thresholdvalue of the concentration of an unburned hydrocarbon gas for Active OBDcorresponding to the exhaust gas temperature from the threshold data forActive OBD previously stored in the storage portion (step S12). Also inthe case of Active OBD, the threshold value is previously set in alltemperatures within a possible temperature range of the oxidationcatalyst 600 (approximately −40° C. to 1000° C.). Since there is not aparticular restriction on how to give the threshold value, the thresholdvalue may be given as a continuous function of a temperature of theoxidation catalyst 600 (exhaust gas temperature) or given as a fixedvalue for every temperature range.

Next, the electronic control device 200 sends a fuel-injectioninstructing signal sg1 to the fuel injection valve 301 to inject anextremely small amount of fuel for a short time (step S13). Morespecifically, such fuel injection is executed at the timing ofpost-injection in an engine cycle of the engine main body 300. Further,a unit injection amount is preferably 1 to 3 (mg/injection), aninjection time is preferably 1 to 6 (sec), and a total injection amountis preferably 15 to 130 (mg). Particularly, the total injection amountis preferably 50 to 65 (mg). In such a case, it becomes possible tosuitably perform the diagnosis by Active OBD while suppressing a fuelinjection amount. In addition, the total injection amount is calculatedfrom the following (Formula 2).

Total injection amount (mg)=Unit injection amount(mg/injection)×Injection time (sec)×engine speed (rpm)/120  (Formula 2)

Further, main examples of the total injection amount are shown in Table1.

TABLE 1 injection total fuel amount injection exhaust gas engine speed[mg/ injection time amount temperature [rpm] injection] [sec] [mg] 150°C. 2000 1 6 100 1 3 17 3 1 50 200° C. 2500 1 6 125 1 3 21 3 1 63 DPFregeneration 2000 6 150 15000 mode (150° C.)

As shown in Table 1, when the engine speed is 2000 rpm at which theexhaust gas temperature becomes about 150° C., and when the engine speedis 2500 rpm at which the exhaust gas temperature becomes about 200° C.,a value of each total injection amount is in the range of 50 to 65 (mg)in the case of a unit injection amount of 3 (mg/injection) and aninjection time of 1 (sec).

For comparison, Table 1 describes an example of fuel injection targetedfor a DPF (diesel particulate filter), which is usually disposed in anexhaust pipe of an automobile, for regeneration of the DPF (DPFregeneration mode). As shown in Table 1, when such a DPF regenerationmode is performed in the situation in which the engine speed is 2000 rpmat which the exhaust gas temperature becomes about 150° C., injection ofa unit injection amount of about 6 (mg/injection) is executed in aninjection time of about 150 (sec). The total injection amount of such acase is about 15000 (mg). Since this value is several hundreds timeslarger than the total injection amount of the diagnosis by Active OBD inthe preferred embodiment, a fuel consumption in Active OBD in thepreferred embodiment can be said to be practically extremely small.

When such an extremely small amount of fuel injection is executed,injected fuel is vaporized inside of the engine main body 300 anddischarged to the exhaust pipe 500 in a superimposed manner in additionto the exhaust gas G steadily discharged in association with anoperation of the engine main body 300, and therefore an unburnedhydrocarbon gas at a concentration higher than a steady condition issent to the oxidation catalyst 600.

Then, at the timing linked with the fuel injection, more specifically,at the timing when the exhaust gas G containing a high concentration ofan unburned hydrocarbon gas generated in association with the fuelinjection reaches the oxidation catalyst 600, adsorption or oxidation ofthe unburned hydrocarbon gas in the exhaust gas G takes place inside theoxidation catalyst 600, and the rest thereof is discharged to thedownstream side, as with Passive OBD, the concentration Nl of anunburned hydrocarbon gas on a downstream side of the oxidation catalyst600 is measured in the HC sensor 100 (step S14).

Subsequent procedures are the same as in Passive OBD. That is, aftermeasurement of the concentration Nl of an unburned hydrocarbon gas on adownstream side and call of the threshold value, the electronic controldevice 200 compares the measured value to the threshold value (stepS15), and when the former is larger than the latter (YES in the stepS15), the electronic control device 200 determines that deteriorationoccurs to a controversial extent (requiring replacement, etc.) in theoxidation catalyst 600 (to be NG) (step S16), and when the latter islarger than the former (NO in the step S15), the electronic controldevice 200 determines that such deterioration does not occur (to be OK)(step S17).

When diagnosis is repeated after the diagnosis regardless of whetherdetermined to be NG or to be OK (YES in the step S18), this process isrepeated again from checking an exhaust gas temperature by thetemperature sensor 110. If not so, the diagnosis is ended as it is (NOin the step S18).

<Detail of Diagnosis by Active OBD>

Characteristic of the diagnosis by Active OBD according to the procedureshown in FIG. 13 will be described based on FIG. 14 to FIG. 18.

First, FIG. 14 is a view showing a relation between the concentration Nuof an unburned hydrocarbon gas on an upstream side and the concentrationNl of an unburned hydrocarbon gas on a downstream side in severalconversion rates in the case where a temperature of an oxidationcatalyst 600 is in an oxidation region (hereinafter, the temperature ofthe oxidation catalyst 600 in this case is denoted by Tc). For example,Tc is 200° C. In FIG. 14, a relation between the concentration Nu of anunburned hydrocarbon gas on an upstream side and the concentration Nl ofan unburned hydrocarbon gas on a downstream side is indicated by a thinsolid line, a broken line and a thick solid line for the conversionrates of 90%, 80% and 70%, respectively. In addition, when thetemperature is Tc, the oxidation catalyst having a conversion rate of90% is assumed as a fresh article.

On the other hand, FIG. 15 is a view showing a relation between theconcentration Nu of an unburned hydrocarbon gas on an upstream side andthe concentration Nl of an unburned hydrocarbon gas on a downstream sidein several conversion rates in the case where a temperature of anoxidation catalyst 600 is in an adsorption region (hereinafter, thetemperature of the oxidation catalyst 600 in this case is denoted byTd). For example, Td is 150° C. In FIG. 15, a relation between theconcentration Nu of an unburned hydrocarbon gas on an upstream side andthe concentration Nl of an unburned hydrocarbon gas on a downstream sideis indicated by a thin solid line, a broken line and a thick solid linefor the conversion rates of 50%, 30% and 10%, respectively. In addition,when the temperature is Td, the oxidation catalyst having a conversionrate of 50% is assumed as a fresh article.

In FIG. 14 and FIG. 15, a range of the axis is extended to a higherconcentration side than those of FIG. 11 and FIG. 12; however, theconversion rate in the oxidation catalyst 600 absolutely obeys Formula1, and therefore the concentration Nl of an unburned hydrocarbon gas ona downstream side has a linearity with respect to the concentration Nuof an unburned hydrocarbon gas on an upstream side even when theconcentration Nu of an unburned hydrocarbon gas on an upstream side isincreased. Thus, as shown in FIG. 14 and FIG. 15, magnitude of theconversion rate in the oxidation catalyst 600 is represented bymagnitude of the slope of a straight line, as with FIG. 11 and FIG. 12.

The diagnosis by Active OBD is common to the diagnosis by Passive OBD inthat the determination of NG or OK depends on whether the concentrationNl of an unburned hydrocarbon gas on a downstream side exceeds thethreshold value previously set or not.

However, in the case of diagnosis by Active OBD, since the diagnosis isexecuted after injecting fuel, the concentration Nu of an unburnedhydrocarbon gas on an upstream side during executing the diagnosisbecomes higher than the variation range R1 of upstream HC concentrationwhich is used in performing the diagnosis by Passive OBD.

FIG. 16 and FIG. 17 are views each illustrating a change of an outputvalue from the HC sensor 100 (HC sensor output) with respect to a timein performing diagnosis by Active OBD on a fresh article and an agedarticle as targets. FIG. 16 is a view in the case that a temperature ofthe oxidation catalyst 600 is 200° C. belonging to the oxidation regionand FIG. 17 is a view in the case that a temperature of the oxidationcatalyst 600 is 150° C. belonging to the adsorption region. With respectto a fuel injection condition, for the former, the unit injection amountwas 1 (mg/injection) and the injection time was 6 (sec), and for thelatter, the unit injection amount was 3 (mg/injection) and the injectiontime was 1 (sec).

The fuel injection is executed at the point in a horizontal axis where atime is about 20 seconds in any of FIG. 16 and FIG. 17, and then the HCsensor output increases several seconds later. The electronic controldevice 200 acquires the HC sensor output for the diagnosis by Active OBDat the timing when the output value increases. In addition, the verticalaxis of FIG. 16 and FIG. 17 is “mV” being a unit of a voltage valuewhich is an actual HC sensor output. In performing Active OBD, asdescribed above, a concentration value is computed from the HC sensoroutput thus represented as the voltage value, and is compared with thethreshold value.

Considering that the HC sensor output is largely fluctuated in executingActive OBD as shown in FIGS. 16 and 17, in FIG. 14 and FIG. 15, apossible concentration range of the concentration Nu of an unburnedhydrocarbon gas on an upstream side in the case of such Active OBD isrepresented as a variation range R2 of upstream HC concentration duringfuel injection. That is, in the diagnosis by Active OBD, when theconcentration Nu of an unburned hydrocarbon gas on an upstream side lieswithin such a variation range R2 of upstream HC concentration, ameasured value of the concentration Nl of an unburned hydrocarbon gas ona downstream side measured by the HC sensor 100 is compared to apredetermined threshold value.

In FIG. 14 and FIG. 15, a range of at least 3850 ppm and at most 4150ppm is illustrated as the variation range R2 of upstream HCconcentration during fuel injection. In addition, it is previously foundempirically that with respect to the variation range R2 of upstream HCconcentration during fuel injection, its central value and adifferential value between the central value and upper/lower limit valuecan be identified according to the conditions of fuel injection. In thecase of being illustrated in FIG. 14 and FIG. 15, a range of ±150 ppmcentered at 4000 ppm is defined as the variation range R2 of upstream HCconcentration during fuel injection.

As described above, in the case of the diagnosis by Active OBD, sincethe concentration range of the concentration Nu of an unburnedhydrocarbon gas on an upstream side as a target is higher than that inthe diagnosis by Passive OBD, the threshold value of the concentrationNl of an unburned hydrocarbon gas on a downstream side used inperforming the diagnosis, is higher than that in the diagnosis byPassive OBD.

For example, FIG. 14 illustrates the case in which the threshold valuesin performing the diagnoses by Passive OBD and Active OBD in the case ofthe temperature Tc are set to 300 ppm and 1000 ppm, respectively. Inthis case, the oxidation catalyst 600 having the conversion rate of 70%is certainly determined to be NG in Active OBD since a measured value ofthe concentration Nl of an unburned hydrocarbon gas on a downstream sideexceeds 1000 ppm in a range of the variation range R2 of upstream HCconcentration.

Further, FIG. 15 illustrates the case in which the threshold values inperforming the diagnoses by Passive OBD and Active OBD in the case ofthe temperature Td are set to 700 ppm and 3000 ppm, respectively. Inthis case, the oxidation catalyst 600 having the conversion rate of 10%is certainly determined to be NG in Active OBD since a measured value ofthe concentration Nl of an unburned hydrocarbon gas on a downstream sideexceeds 3000 ppm in a range of the variation range R2 of upstream HCconcentration.

To be safe, in the case of the temperature Tc shown in FIG. 14, theoxidation catalyst 600 having the conversion rate of 70%, which iscertainly determined to be NG in Active OBD, is certainly determined tobe OK when performing Passive OBD in which the threshold value is set to300 ppm.

Similarly, in the case of the temperature Td shown in FIG. 15, theoxidation catalyst 600 having the conversion rate of 10%, which iscertainly determined to be NG in Active OBD, can be determined to be OKor to be NG depending on an actual value of the concentration Nu of anunburned hydrocarbon gas on an upstream side when performing Passive OBDin which the threshold value is set to 700 ppm.

Considering that in the former, the catalyst is certainly determined tobe NG in performing Passive OBD only when the conversion rate is lessthan 20%, and in the latter, no catalyst is certainly determined to beNG in Passive OBD, it can be said that in accordance with the ActiveOBD, the diagnosis of the degree of deterioration can be performed withhigher accuracy (on the severe basis) than the Passive OBD. This is byvirtue of the effect that, in Active OBD, since a value of theconcentration Nu of an unburned hydrocarbon gas on an upstream side islarge, a value of the concentration Nl of an unburned hydrocarbon gas ona downstream side inevitably becomes large, and therefore a differencein the conversion rate exerts a large influence upon a difference inmeasurement value in the HC sensor 100.

These means that the deterioration of the oxidation catalyst 600 can bediagnosed with accuracy by using the diagnosis by Passive OBD inconjunction with the diagnosis by Active OBD while setting therespective threshold values appropriately.

In addition, such a combined use is particularly effective when thetemperature of the oxidation catalyst 600 is in the oxidation region.This is because, since the conversion rate of the oxidation catalyst 600whose temperature is in the oxidation region is relatively high even ifthe catalyst 600 is an aged article as distinct from the case in whichthe temperature is in the adsorption region and particularly in the caseof steady operation, a value of the concentration Nu of an unburnedhydrocarbon gas on an upstream side is small, in many cases, adifferential HC sensor output between the fresh article and the agedarticle in performing Passive OBD in the oxidation region (e.g., Δ1 inFIG. 16) tends to be small, but a differential HC sensor output betweenthe fresh article and the aged article in performing Active OBD (e.g.,Δ2 in FIG. 16) becomes large.

Incidentally, also when a temperature of the oxidation catalyst 600 isin the adsorption region, a differential HC sensor output between thefresh article and the aged article in performing Active OBD (e.g., Δ4 inFIG. 17) tends to be larger than a differential HC sensor output betweenthe fresh article and the aged article in performing Passive OBD (e.g.,Δ3 in FIG. 17), but a difference between the differential HC sensoroutput in Passive OBD and the differential HC sensor output in ActiveOBD is smaller than the difference in the oxidation region since anabsolute value of Δ3 is relatively large in the first place.

Further, in Active OBD, there is also an effect obtained by varying thetotal injection amount in injecting fuel. For example, in the case ofthe temperature Tc shown in FIG. 14, a variation range R3 of upstream HCconcentration is defined in a range of at least 5000 ppm and at most5300 ppm which is higher in concentration than the variation range R2 ofupstream HC concentration. When the threshold value is 1000 ppm as withthe case of using the above-mentioned variation range R2 of upstream HCconcentration as a target, the oxidation catalyst 600 having theconversion rate less than 80% which can be determined to be OK in thecase of using the variation range R2 of upstream HC concentration as atarget is certainly determined to be NG. This means that by using thevariation range R2 of upstream HC concentration in conjunction with thevariation range R3 of upstream HC concentration, it is possible todiscriminate among the oxidation catalyst 600 with the conversion rateless than 70%, the oxidation catalyst 600 with the conversion rate of atleast 70% and less than 80% and the oxidation catalyst 600 with theconversion rate of at least 80%.

Similarly, in the case of the temperature Td shown in FIG. 15, avariation range R4 of upstream HC concentration is defined in a range ofat least 5350 ppm and at most 5650 ppm which is higher in concentrationthan the variation range R2 of upstream HC concentration. When thethreshold value is 3000 ppm as with the case of using theabove-mentioned variation range R2 of upstream HC concentration as atarget, the oxidation catalyst 600 having the conversion rate less than30% is also determined to be NG.

When performing multiple Active OBDs in which the total injection amountin injecting fuel is thus varied, this enables more detaileddeterioration diagnosis which cannot be achieved by Passive OBD.

FIG. 18 is a view showing the results of analyses of components in theexhaust gases G by a gas analyzer in acquiring HC sensor outputs shownin FIG. 16 on the fresh article and the aged article.

While the HC sensor output value varies in FIG. 16 as a result of fuelinjection performed for Active OBD, variation corresponding to fuelinjection occurs also in FIG. 18. However, in FIG. 18, only a graph ofTHC being a typical hydrocarbon gas excluding CO of gas componentscontained in the unburned hydrocarbon gas in the aged article, largelyvaries in response to the fuel. It is assumed from such a result thatthe deterioration of oxidation of the aged article in the oxidationregion is mainly due to the deterioration of oxidizing capability to thehydrocarbon. In addition, it is known that temperature dependency ofcatalytic ability of the oxidation catalyst 600 is different dependingon a gas component as a target, and the result shown in FIG. 18 mayconfirm this. Such a result suggests that it is possible to determinethe degree of deterioration of oxidizing capability to a specific gascomponent when the temperature of the oxidation catalyst 600 isintentionally varied in executing Active OBD in the oxidation region.

As described above, in accordance with the oxidation catalyst diagnosissystem of the preferred embodiment, it is possible to diagnose, in realtime and with excellent accuracy, the degree of deterioration ofcatalytic ability in an oxidation catalyst, which is disposed at amidpoint in the exhaust pipe from the engine main body being a dieselengine in an engine system, and oxidizes or adsorbs an unburnedhydrocarbon gas in an exhaust gas. It is implemented without calculatingthe conversion rate being an index of the catalytic ability, based onthe concentration of an unburned hydrocarbon gas at a downstreamposition close to the oxidation catalyst, which is directly measured bya hydrocarbon gas sensor disposed at the position in the exhaust pipe.

Particularly, according to Passive OBD in which the diagnosis isperformed by directly using conditions where the engine main body isunder an ordinary operation state, the deterioration of an oxidationcatalyst can be diagnosed at any timing during the operation of theengine main body and without consuming fuel for diagnosis. Further, byrepeating the diagnosis, the accuracy of diagnosis can be more enhanced.

Further, in accordance with Active OBD performing the diagnosis in astate in which the concentration of the unburned hydrocarbon gas flowninto an oxidation catalyst is intentionally increased by injecting anextremely small amount of fuel, the deterioration of the oxidationcatalyst can be diagnosed with higher accuracy than Passive OBD.

Second Preferred Embodiment

In the oxidation catalyst diagnosis system DS1 of the above-mentionedfirst preferred embodiment, the degree of deterioration of the oxidationcatalyst 600 is diagnosed without actually calculating the conversionrate; however, in the preferred embodiment, an aspect in which thedegree of deterioration of the oxidation catalyst 600 is diagnosed byactually calculating the conversion rate, will be described.

FIG. 19 is a view schematically showing a schematic configuration of anengine system 2000 configured to include an oxidation catalyst diagnosissystem DS2 of second preferred embodiment of the present invention.

The oxidation catalyst diagnosis system DS2 of the preferred embodimentand the engine system 2000 have the same configurations as in theoxidation catalyst diagnosis system DS1 and the engine system 1000 offirst preferred embodiment except that the HC sensor 100A and the HCsensor 100B are respectively disposed at both of an upstream and adownstream positions close to the oxidation catalyst 600 serving as adiagnosis target in the oxidation catalyst diagnosis system DS2 in theexhaust pipe 500. Thus, in the preferred embodiment, description aboutthe detail of each constituent will be omitted. In the oxidationcatalyst diagnosis system DS2, the HC sensor 100B disposed at adownstream positions close to the oxidation catalyst 600 sends a HCdetection signal sg11 to the electronic control device 200 as with theHC sensor 100 of first preferred embodiment, and the HC sensor 100Adisposed at a upstream positions close to the oxidation catalyst 600also sends a HC detection signal sg21 to the electronic control device200.

FIG. 20 is a view showing an example of a procedure of performingPassive OBD by an oxidation catalyst diagnosis system DS2.

The diagnosis starts with checking a temperature of the exhaust gas G(exhaust gas temperature) in the vicinity on an upstream side of theoxidation catalyst 600 by the temperature sensor 110, as with PassiveOBD in first preferred embodiment (step S21). More specifically, theelectronic control device 200 acquires exhaust gas temperature detectionsignals sg12 sent from the temperature sensor 110 and thereby theexhaust gas temperature is identified. Such an exhaust gas temperatureis regarded as a temperature of the oxidation catalyst 600 at the pointin time.

Subsequently, the electronic control device 200 calls the thresholdvalue of the conversion rate corresponding to the exhaust gastemperature from the threshold data for Passive OBD previously stored inthe storage portion (step S22). In the preferred embodiment, thethreshold value related to the conversion rate is previously set in alltemperatures within a possible temperature range of the oxidationcatalyst 600 (approximately −40° C. to 1000° C.). Further, in thepreferred embodiment, the lower limit value of the allowable conversionrate range becomes the threshold value as it is. Since there is not aparticular restriction on how to give the threshold value, the thresholdvalue may be given as a continuous function of a temperature of theoxidation catalyst 600 (exhaust gas temperature) or given as a fixedvalue for every temperature range. In addition, the threshold value insuch a case is set in view of having temperature dependency as shown inFIG. 3 in the catalytic ability of the oxidation catalyst 600. Usually,the threshold value in the oxidation region is set larger than thethreshold value in the adsorption region.

Next, the concentration Nu of an unburned hydrocarbon gas on an upstreamside and the concentration Nl of an unburned hydrocarbon gas on adownstream side of the oxidation catalyst 600 are measured by two HCsensors 100A and 100B (step S23). More specifically, measurement by a HCsensor 100A is performed immediately before the exhaust gas G whosetemperature has been measured by the temperature sensor 110 reaches theoxidation catalyst 600, adsorption or oxidation of the unburnedhydrocarbon gas in the exhaust gas G takes place in the oxidationcatalyst 600, and then the rest of the unburned hydrocarbon gas isdischarged to the downstream side, and at this timing, measurement by aHC sensor 100B is performed.

FIG. 21 and FIG. 22 are each a view illustrating a HC sensor output in afresh article and an aged article respectively, in the case where atemperature of the oxidation catalyst 600 is 200° C. It is verified fromFIG. 21 and FIG. 22 that the aged article is larger in a differencebetween an upstream output and a downstream output than the fresharticle.

After acquiring measured values in two HC sensors 100A and 100B, theelectronic control device 200 calculates a conversion rate based onFormula 1 using these measured values (step S24).

Then, the electronic control device 200 compares the calculatedconversion rate to the threshold value (step S25), and when the formeris larger than the latter (YES in the step S25), the electronic controldevice 200 determines that deterioration occurs to a controversialextent (requiring replacement, etc.) in the oxidation catalyst 600 (tobe NG) (step S26), and when the latter is larger than the former (NO inthe step S25), the electronic control device 200 determines that suchdeterioration does not occur (to be OK) (step S27).

When diagnosis is repeated after the diagnosis regardless of whetherdetermined to be NG or to be OK (YES in the step S28), this process isrepeated again from checking an exhaust gas temperature by thetemperature sensor 110. If not so, the diagnosis is ended as it is (NOin the step S28).

As described above, in the case of Passive OBD in the preferredembodiment, since the conversion rate is actually calculated and thepresence or absence of controversial deterioration is diagnosed,diagnosis having higher reliability than Passive OBD in first preferredembodiment can be performed. For example, in FIG. 20, one conversionrate is compared with one threshold value; however, it may be employedthat the threshold value is set in multi-stage to diagnose the degree ofdeterioration of the oxidation catalyst 600.

Also in the oxidation catalyst diagnosis system DS2 of the preferredembodiment, the diagnosis by Active OBD can be performed in terms of theconfiguration as with the oxidation catalyst diagnosis system DS1 offirst preferred embodiment, but it can be said that since the diagnosisby Passive OBD can be performed with accuracy, as described above, amerit of performing Active OBD associated with fuel injection althoughan extremely small amount, positively is smaller than that in firstpreferred embodiment.

As described above, in accordance with the oxidation catalyst diagnosissystem of the preferred embodiment, since hydrocarbon gas sensors aredisposed on both of the upstream and the downstream of the oxidationcatalyst, it is possible to determine whether controversialdeterioration occurs in an oxidation catalyst or not, based on theresult of calculating the conversion rate of the oxidation catalystbased on measured values of both hydrocarbon gas sensors during anordinary operation of the engine main body, and comparing the calculatedvalue with a predetermined threshold value, and therefore it becomespossible to perform highly reliable diagnosis in Passive OBD notexecuting fuel injection.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

What is claimed is:
 1. A method for diagnosing a degree of deteriorationof a catalyst, said catalyst being disposed in an exhaust path of aninternal combustion engine and oxidizes or adsorbs a target gas,including at least one of a hydrocarbon gas and a carbon monoxide gas,contained in an exhaust gas from said internal combustion engine, saidmethod comprising the steps of: a) detecting the concentration of saidtarget gas downstream from said catalyst in said exhaust path at anytiming when said internal combustion engine is in a state of a steadyoperation; and b) determining whether deterioration exceeding anacceptable level of said catalyst occurs or not by comparing theconcentration of said target gas detected in said step a) with athreshold value of the concentration of said target gas corresponding tothe temperature of said catalyst at said timing, wherein said thresholdvalue is previously defined according to an allowable range of an indexvalue representing the degree of oxidation or adsorption at saidcatalyst corresponding to the temperature of said catalyst at saidtiming.
 2. The catalyst deterioration diagnosis method according toclaim 1, wherein when the concentration of said target gas in thevicinity on an upstream side of said catalyst in said exhaust path isdenoted by Nu, the concentration of said target gas in the vicinity on adownstream side of said catalyst in said exhaust path is denoted by Nl,and a conversion rate defined by the following calculation formula isused as an index value representing the degree of oxidation oradsorption occurring in said catalyst,Conversion rate (%)=100×(Nu−Nl)/Nu, an allowable conversion rate rangebeing a range of said conversion rate allowable in said catalyst ispreviously defined according to a possible temperature of said catalyst,a value of said Nl with respect to the possible temperature of saidcatalyst, which is calculated by substituting, into said calculationformula, a lower limit value of said allowable conversion rate range atthe temperature as said conversion rate, and an upper limit value of arange of an upstream gas concentration being a possible concentrationrange of said target gas in said vicinity on an upstream side of saidcatalyst during a steady-operation state of said internal combustion assaid Nu, is defined as said threshold value, and said catalystdeterioration diagnosis method further comprises the steps of: c)measuring a temperature of said exhaust gas containing said target gasat said upstream side at said any timing when said internal combustionengine is in said state of a steady operation; d) detecting said targetgas at the downstream from said catalyst in said exhaust path at thetiming when said exhaust gas whose temperature has been measured in saidstep c) is discharged from said catalyst to calculate the concentrationof said target gas at said downstream side based on the detectedresults; and e) diagnosing the degree of deterioration of said catalyst,based on the concentration of said target gas at said downstream sidecalculated in said step d), the temperature of said catalyst identifiedbased on a measurement value in said step c), and said threshold valueat the temperature of the catalyst, wherein in said step e),deterioration exceeding an acceptable level of a catalyst is determinednot to occur in said catalyst when the concentration of said target gascalculated in said step d) is said threshold value or less, anddeterioration exceeding an acceptable level of a catalyst is determinedto occur in said catalyst when the concentration of said target gascalculated in said step d) is larger than said threshold value.
 3. Thecatalyst deterioration diagnosis method according to claim 1, whereinwhen the concentration of said target gas in the vicinity on an upstreamside of said catalyst in said exhaust path is denoted by Nu, theconcentration of said target gas in the vicinity on a downstream side ofsaid catalyst in said exhaust path is denoted by Nl, and a conversionrate defined by the following calculation formula is used as an indexrepresenting the degree of oxidation or adsorption occurring in saidcatalyst,Conversion rate (%)=100×(Nu−Nl)/Nu, an allowable conversion rate rangebeing a range of said conversion rate allowable in said catalyst ispreviously defined according to a possible temperature of said catalyst,a value of said Nl with respect to the possible temperature of saidcatalyst, which is calculated by substituting, into said calculationformula, a lower limit value of said allowable conversion rate range atthe temperature as said conversion rate, and an upper limit value of arange of an upstream gas concentration being a possible concentrationrange of said target gas in said vicinity on an upstream side of saidcatalyst during a steady-operation state of said internal combustion assaid Nu, is defined as said threshold value, and said catalystdeterioration diagnosis method repeats the following steps in order: c)measuring a temperature of said exhaust gas containing said target gasat said upstream side at said any timing when said internal combustionengine is in said state of a steady operation; d) detecting said targetgas at the downstream from said catalyst in said exhaust path at thetiming when said exhaust gas whose temperature has been measured in saidstep c) is discharged from said catalyst to calculate the concentrationof said target gas at said downstream side based on the detectedresults; and e) diagnosing the degree of deterioration of said catalyst,based on the concentration of said target gas at said downstream sidecalculated in said step d), the temperature of said catalyst identifiedbased on a measurement value in said step c), and said threshold valueat the temperature of the catalyst, wherein in said step e),deterioration exceeding an acceptable level of a catalyst is determinednot to occur in said catalyst when the number of the cases where theconcentration of said target gas calculated in said step d) is saidthreshold value or less, is a predetermined reference number or less,and in said step e), deterioration exceeding an acceptable level of acatalyst is determined to occur in said catalyst when the number of thecases where the concentration of said target gas calculated in said stepd) is said threshold value or less, is larger than a predeterminedreference number.
 4. A method for diagnosing a degree of deteriorationof a catalyst, said catalyst being disposed in an exhaust path of aninternal combustion engine and oxidizes or adsorbs a target gas,including at least one of a hydrocarbon gas and a carbon monoxide gas,contained in an exhaust gas from said internal combustion engine, saidmethod comprising the steps of: a) detecting said target gas at theupstream and the downstream from said catalyst in said exhaust path atany timing when said internal combustion engine is in a state of asteady operation to calculate the concentration of said target gas; andb) determining whether deterioration exceeding an acceptable level ofsaid catalyst occurs or not by comparing an index value representing thedegree of oxidation or adsorption at said catalyst with a thresholdvalue related to said index value corresponding to the temperature ofsaid catalyst at said timing, based on the calculated results obtainedin said step a).
 5. The catalyst deterioration diagnosis methodaccording to claim 4, wherein when the concentration of said target gasin the vicinity on an upstream side of said catalyst in said exhaustpath is denoted by Nu, the concentration of said target gas in thevicinity on a downstream side of said catalyst in said exhaust path isdenoted by Nl, and a conversion rate defined by the followingcalculation formula is used as an index value representing the degree ofoxidation or adsorption occurring in said catalyst,Conversion rate (%)=100×(Nu−Nl)/Nu, an allowable conversion rate rangebeing a range of said conversion rate allowable in said catalyst ispreviously defined according to a possible temperature of said catalyst,a lower limit value of said allowable conversion rate range with respectto the possible temperature of said catalyst is defined as saidthreshold value, and said catalyst deterioration diagnosis methodfurther comprises the steps of: c) measuring a temperature of saidexhaust gas containing said target gas at said upstream side at said anytiming when said internal combustion engine is in a state of a steadyoperation; d) detecting said target gas at the upstream from saidcatalyst in said exhaust path immediately before said exhaust gas whosetemperature has been measured in said step c) reaches said catalyst tocalculate the concentration of said target gas at said upstream sidebased on the detected results; e) detecting said target gas at thedownstream from said catalyst in said exhaust path at the timing whensaid exhaust gas whose temperature has been measured in said step c) isdischarged from said catalyst to calculate the concentration of saidtarget gas at said downstream side based on the detected results; f)calculating a conversion rate in said catalyst from concentration valuesof said target gases on an upstream side and a downstream of saidcatalyst calculated in said first and second gas concentrationcalculation steps; and g) diagnosing the degree of deterioration of saidcatalyst, based on the conversion rate calculated in said step f), thetemperature of said catalyst identified based on a measurement value insaid step c), and said threshold value at the temperature of thecatalyst, wherein in said step g), deterioration exceeding an acceptablelevel of a catalyst is determined not to occur in said catalyst when theconversion rate calculated in said step f) is said threshold value ormore, and deterioration exceeding an acceptable level of a catalyst isdetermined to occur in said catalyst when the concentration of saidtarget gas calculated in said step f) is smaller than said thresholdvalue.
 6. The catalyst deterioration diagnosis method according to claim1, wherein a mixed-potential hydrocarbon gas sensor is used fordetection of said target gas, said mixed-potential hydrocarbon gassensor comprising a sensing electrode whose catalyst activity isdisabled with said sensing electrode being formed of a Pt—Au alloy. 7.The catalyst deterioration diagnosis method according to claim 2,wherein a mixed-potential hydrocarbon gas sensor is used for detectionof said target gas, said mixed-potential hydrocarbon gas sensorcomprising a sensing electrode whose catalyst activity is disabled withsaid sensing electrode being formed of a Pt—Au alloy.
 8. The catalystdeterioration diagnosis method according to claim 3, wherein amixed-potential hydrocarbon gas sensor is used for detection of saidtarget gas, said mixed-potential hydrocarbon gas sensor comprising asensing electrode whose catalyst activity is disabled with said sensingelectrode being formed of a Pt—Au alloy.
 9. The catalyst deteriorationdiagnosis method according to claim 4, wherein a mixed-potentialhydrocarbon gas sensor is used for detection of said target gas, saidmixed-potential hydrocarbon gas sensor comprising a sensing electrodewhose catalyst activity is disabled with said sensing electrode beingformed of a Pt—Au alloy.
 10. The catalyst deterioration diagnosis methodaccording to claim 5, wherein a mixed-potential hydrocarbon gas sensoris used for detection of said target gas, said mixed-potentialhydrocarbon gas sensor comprising a sensing electrode whose catalystactivity is disabled with said sensing electrode being formed of a Pt—Aualloy.